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DEPARTMENT OF COMMERCE 



Technologic Papers 



OP THE 



Bureau of Standards 

S. W.8TRATTON, Director H 



STRUCTURE OF THE COATING ON TINNED SHEET 
COPPER IN RELATION TO A SPECIFIC 

CASE: OF CORROSION HHI 



BY 

PAUL D. MERICA, Associated Physicist 

EfeSM Bureau of Standards 



ISSUED £PRIL 21, 1917 




WASHINGTON 
GOVERNMENT PRINTING OFIICH 
1817 I H 



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DEPARTMENT OF COMMERCE 



Technologic Papers 



OF THE 



Bureau of Standards 

S. W. STRATTON, Director 



No. 90 

STRUCTURE OF THE COATING ON TINNED SHEET 

COPPER IN RELATION TO A SPECIFIC 

CASE OF CORROSION 



BY 

PAUL D. MERICA, Associate Physicist 
ii 
Bureau of Standards 



ISSUED APRIL 21, 1917 




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1917 






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STRUCTURE OF THE COATING ON TINNED SHEET COPPER 
IN RELATION TO A SPECIFIC CASE OF CORROSION 



By Paul D. Merica 



CONTENTS 

Page 

Introduction 3 

I. Pitting of tinned copper roofing sheet 3 

II. Manufacture of tinned sheet copper 5 

III. Structure of tin coatings on copper 6 

IV. Electrolytic potential and corrodibility of the constituents of the tin coating 10 
V. Discussion and conclusions 15 

INTRODUCTION 

There has recently come to the attention of the Bureau of 
Standards an instance of corrosion of tinned sheet copper, which 
presents some rather unusual and interesting features, in the inves- 
tigation of which a study has been made of the structure and prop- 
erties of tin coatings on copper, with particular reference to the 
influence of this structure on the resistance of this material to 
corrosion. 

This question of the corrosion of such material is a fairly im- 
portant one, although in comparison to the quantity of other coated 
materials, such as tin plate and galvanized iron, commercially 
used, that of tinned copper is small. It finds, however, fairly wide 
application, being used for roofing, for containing vessels such as 
milk cans, and for fittings, troughs, etc., for soda fountains and 
breweries. It may be noted that of the total amount of copper 
roofing used only a small proportion, perhaps 5 per cent, is tinned. 
In many cases the application of tin is made as much for the sake 
of the appearance of the article as for the protection afforded 
against corrosion. 

I. PITTING OF TINNED COPPER ROOFING SHEET 

The greater part of the roof of the Library of Congress, in Wash- 
ington, is covered with tinned sheet copper material, all of which 
is from the same manufacturer, and which was installed at the 
time of the completion of the building, approximately 1893 to l8 94- 

3 



4 Technologic Papers of the Bureau of Standards 

This is 16-ounce sheet, tinned on both sides, and it lies on cement 
or terra cotta. 

The lower or under side of this sheet has become but little 
altered in the course of time, except that it has become blackened 
at points where the sheet is perforated by the holes described 
below. The upper or exposed side has taken on, for the most part, 
a dark, slightly greenish patina, which is dense and coherent, 
although in some areas there is a whitish tinge, due to the tin or 
tin oxide. On this surface are found in fairly dense distribution 
small pits or furrows, of which the diameter or width varies from 
0.5 to i mm, the furrows being sometimes as much as 5 cm long. 
The appearance of these pits and furrows is shown in Fig. 1. In 
many sheets there will be as many as 20 or 25 per square decimeter; 
in others much fewer. Often these extend completely through the 
sheet as perforations, through which leakage takes place. The 
edges of these pits are exceedingly sharp and well defined, much 
more so than the photograph indicates. Their inside surfaces are 
sometimes covered with a thin reddish or black layer of copper 
oxide, often also, with a thin green layer, which is probably basic 
carbonate of copper. It was stated that these pits had first been 
noticed some 8 or 10 years after completion of the roof. 

Careful examination fails to relate the location or distribution 
of these pits to any features of service conditions, such as prox- 
imity to ventilators, chimneys, or to places where soot or cinders 
accumulate. They occur practically in all of the sheets, both in 
the gutters and on the slopes. They are much less serious in 
extent on north exposures and in areas within shadow; on the 
vertical surfaces where the sheets are joined by flanging they are 
practically absent. These pits are also not arranged in any man- 
ner symmetrical to the direction of rolling of the sheet. 

It is to be noted that the Library of Congress is located in a 
section of Washington, itself uncommonly free from smoke, etc., 
which is not near power stations or factories producing smoke, 
such that the atmospheric conditions may be looked on as most 
unfavorable for corrosion. 

The most striking feature about the pits is their distribution 
almost without exception along the line of surface scratches. This 
can be seen clearly in Fig. 1. They generally occur in groups, the 
scratch passing through the center of each pit, although some 
isolated pits are found through which no such scratches pass; 
these are, however, comparatively rare. It is really this peculiar 



Corrosion of Tinned Sheet Copper 5 

feature which has lent interest and also direction to the present 
work. 

Pitting and other local corrosion of copper and other metals 
and alloys is unfortunately a well-known phenomenon. Refer- 
ences on this topic will be found quite generally. 1 Such local 
corrosion is generally held to be electrolytic in origin, depending 
on the simultaneous presence of electronegative and electroposi- 
tive areas, "hard" and "soft" areas, oxide inclusions, etc., or is 
due to the deposition of electronegative basic salt, such as that of 
zinc oxychloride on brass. In the case of tin plate, such pitting 
is held to be due to the presence of pinholes originating during the 
process of tinning, which allow access of the corroding liquid or 
water to the iron, which is in electric contact with the more electro- 
negative tin. The author has found no reference to local corrosion 
of tinned copper such as has been described above. 

H. MANUFACTURE OF TINNED SHEET COPPER 

Copper sheets are generally tinned in the following manner: 
The tin mixture is melted up in a cast-iron pot; in the meantime 
the copper sheets, after having been cleaned and pickled are 
rubbed with a fluxing solution of zinc chloride and hydrochloric 
acid. They are, then, one by one, laid on an inclined plane adja- 
cent to the pot ; the operator takes a ladle full of molten tin and 
flows it over the sheet, the excess running back by troughs into 
the pot. Then, beginning with the top of the sheet, he wipes off 
the excess tin with a brush or bundle of tow. This produces a 
smooth even coating. 

Another method of tinning is also used. The cleaned and 
pickled sheet is laid on a bench, which contains an inset gas grate, 
flush with the top. The sheet is passed over the grate and heated, 
while the operator takes small flat plates of tin, lays them on the 
sheet, and rubs them into the copper sheet when they have melted. 
Powdered ammonium chloride is used as a flux. The manufac- 
turers claim that this method gives a more uniform coating than 
the first one. 

A tinning mixture is often used containing a small amount of 
lead, as it is claimed that the latter increases the fluidity of the 
mixture and exerts no harmful effect, provided the material is 
not to be used in containers for foodstuffs or liquids for drinking 
purposes. 

1 J. T. Corner, Some Practical Experiences with Corrosion, Jour. Inst. Metals, 5, p.. 115, 1911; T. A. 
Eastick, The Corrosion of Copper, Metal Industry, 11, p. 524, 1913; E. Johnson, Annealing and Diseases of 
Copper, Met. & Chem. Eng., 9, p. 87, 1911. 



6 



Technologic Papers of the Bureau of Standards 



1H. STRUCTURE OF TIN COATINGS ON COPPER 

In order to make a study of the tin coating, other samples 

besides the one mentioned from the Library of Congress were 

obtained. Of these several were most kindly furnished by two 

manufacturers of this material; one was from the roof of the 

statehouse of Texas. These samples are described in the following 

table : 

TABLE 1 

Description of Samples Tested 



B. S. No. 


Sheet 
gage 


1053 


22 


1054 


22 


1119 


22 


1120 


18 


1241 


20 


1242 


17 


1243 


14 


1244 


22 



Furnished by- 



Description 



Library of Congress. 
do 



Manufacturer A.. 

do 

Manufacturer B . . 

do 

do 

Statehouse, Texas . 



New; tinned on one side. 

Corroded and pitted in service; tinned on 

both sides. 
New; tinned on both sides. 

Do. 
New; tinned on one side. 

Do. 

Do. 
Did not pit or corrode in service (30 years). 



A chemical analysis was made of several of the tin coatings. 
A sample of 1054, area 31 cm 2 , was treated with strong hydro- 
chloric acid. The solution resulting contained: 

Grams 
Tin o. 3050 

Copper 2135 

Lead 0269 

Iron 0035 

Zinc 0050 

The coating was computed to contain : 2 

Per cent 

^n 89.5 

Lead 8. o 

Iron 1. o 

Zinc 1. 5 

A sample of 1242 was dissolved in 1 : 1 HC1 with aid of the elec- 
tric current (6 to 8 volts) . The solution contained : 

Grams 

Tin o. 8320 

Copper 6. 8725 

Zinc 0150 

Coating was computed to contain : 2 

Per cent 

Tin 98. 2 

Zinc *■ 77 

Lead Trace (o. 1 per cent). 

Iron Not detected. 

a The copper found in the coating is due to alloying during the tinning operation and is not here included 
in the analysis of the tinning mixture. 



Corrosion of Tinned Sheet Copper 7 

The micro6tructure of a cross section of the base copper of the 
corroded sheet 1054 is shown in Fig. 2 ; the cuprous oxide is clearly 
seen. 

Since the application of the coating has been made at a high 
temperature, above the melting point of tin, 232 ° C, it is to be 
expected that the tin will be found alloyed with the copper at the 



I000 e 



800° 



600° 



400° 



200° 




60% 

Atomic percent Tin 
Fig. 3. — Equilibrium diagram of copper-tin alloys 

juncture of copper and tin, the copper content of the alloy in- 
creasing from the outside to inside of the coating. The equilib- 
rium or constitution diagram of the alloys of copper and tin is 
shown in Fig. 3 as given in Guertler's "Handbuch der Metallo- 
graphies ' 191 2, largely from the work of Hey cock and Neville, 
and modified slightly by Haughton. 3 



3 J. L. Haughton, The Constitution of the Alloys of Copper with Tin, Parts I and II, Jour. Inst. Metals, 
13, p. 222; 1915. 



8 Technologic Papers of the Bureau of Standards 

The ternary diagram of the copper-tin-lead alloys has not yet 
been worked out. It is most probable, judging from the very 
slight solubility of lead in copper and by the fact that in a mixture 
of lead and copper, practically the whole of the lead does not 
solidify above 386 C, that all of the lead in a ternary alloy, con- 
taining only a small percentage of it, will be found in the tin rich 
phase — that is, the eutectic of tin and the constituent X. 

In studying the structure of the coating two methods of pre- 
paring the specimens were used. In order to observe it in cross 
section, samples of the sheet were carefully copperplated, and then 
mounted in ordinary solder, care being taken not to heat the solder 
much above its melting point. It was noticed, however, that 
sometimes, even with care, a certain amount of alloying took 
place at the copper surfaces during the mounting, and therefore 
control specimens were also mounted in plaster of Paris. This 
coating is very thin, and many features of its structure were not 
noticed until it was observed in oblique section. A specimen, 
sometimes, but not always previously copperplated, was bent 
slightly, and then ground and polished through the coating on 
the convex surface. In this way a magnified section of the coating 
was obtained, as it were, which will be hereafter referred to as an 
oblique surface section. This method has been used by Guertler 4 
in studying the structure of zinc coatings on iron. It was found 
that etching to develop structure was best done by the successive 
application of (1) a concentrated solution of ammoniacal copper 
ammonium chloride (Cu-Am-OH-i), and (2) a dilute hydrochloric 
and solution of ferric chloride (FeCl 3 .HCl). 

Fig. 4 shows a transverse section of the coating of 1054 (reverse 
or uncorroded side) . 5 

There are clearly seen (1) the underlying sheet copper; (2) the 
bright, unetched alloy layer; (3) the ground mass of copper-tin 
eutectic, which probably contains also all of the lead, and in 
which crystals of the constituent X can be seen; and (4) the 
protecting layer of copper plate. 

The same layers are also seen in an oblique surface section of 
1054, in Fig. 5. 

Further examination shows that this light, unetched layer next 
to the copper is in reality composed of two constituents. Imme- 
diately adjacent to the copper is found in it a bluish border, which 

4 W. Guertler, Die Struktur des verzinkten Eisens, Int. Zeitsch. Metallographie, 1, p. 352; 1911. 

5 Hereafter photomicrographs and description of the coating structure of 1054 refer, unless otherwise stated, 
to that of the uncorroded side. 



Corrosion of Tinned Sheet Copper 9 

can be readily seen under the microscope but is not easily photo- 
graphed. In order to render it possible of reproduction, it is 
necessary to etch much more heavily with (FeCl 3 .HCl), and until 
all of the eutectic has been attacked and removed. Then this 
blue border darkens and presents the appearance shown in Fig. 6. 

The constituent which darkens very readily with (FeCl 3 .HCl) 
is the eutectic of constituent X and tin, the constituent which 
remains light and unetched is X, the H of Hey cock and Neville, 
containing about 60 per cent by weight of tin ; the blue constituent 
is the VIII, the 77 of Heycock and Neville, and contains about 39 
per cent by weight of tin. 

It has not thus far been possible to isolate still further constitu- 
ents of this coating, although there must be present still one more, 
namely, VII, between the copper and the VIII, and which escapes 
notice probably on account of its extreme thinness. It must be 
borne in mind that the whole coating is generally only about 0.01 
mm thick. 

The structures are here illustrated in a sample (1054) of which 
the coating contains about 8 per cent lead. This has, however, 
in no manner altered the type of structure; the other coatings, 
containing no lead, showed identically the same structures, as 
shown in Figs. 7 and 8. (The VIII constituent can be seen in 
Fig. 8.) In the lead bearing coating th$ lead simply forms part 
of the outer eutectic layer. 

In tinning copper the excess tin runs back into the pot after 
each sheet is tinned. This, however, has dissolved some copper 
and carried it into the pot. In time, if the temperature is low 
enough — under 400 C — crystals are formed, probably of the X 
constituent. These are analogous to galvanizers' dross, the Zn 7 Fe 
crystals, formed in galvanizing iron. When the tin bath becomes 
thus contaminated with copper there is danger of obtaining a 
brittle tin coating; that is, consisting wholly of intermediate alloy 
(X) with no ductile eutectic. 

This coating, of such complicated structure, is not uniform in 
either thickness or structure. The molten tin has etched the 
copper, attacking certain groups of grains more readily than others, 
such that the surface of the copper, after stripping off the coating, 
presents a rippled appearance. This statement applies most 
particularly to 1054. In the ''pockets" or "valleys" of this sur- 
face the excess tin or eutectic has remained, whereas the elevations 
have been wiped off, leaving only a thin coating. The thickness 

72240°— 17 2 



io Technologic Papers of the Bureau of Standards 

of the coating of No. 1054 was, at maximum, about 0.03 mm; 
at minimum, about 0.006 mm; and in average, about 0.012 mm. 

The structure of the coating of 1054 is also quite variable; this 
is illustrated by the Figs. 4, 9-12. The upper copper area is in 
each case the sheet copper, the lower the protecting copper plate, 
marked, respectively, (1) and (4). Fig. 4 shows a "pocket" or 
" valley " area with very thick coating, with continuous alloy layer 
and much eutectic; Fig. 9 shows the average appearance of the 
coating; Fig. 10 shows a break in the alloy layer, the eutectic being 
adjacent, apparently, to the copper; Fig. 1 1 shows an elevated area, 
at which the alloy layer extends through to the surface and is not 
covered by the eutectic; Fig. 12 shows breaks and irregularities in 
the coating. The constituent VIII layer does not vary much in 
thickness; with the exception of a few complete breaks in it, it 
averages about 0.002 mm in thickness. The constituent X is 
more variable in thickness, varying from 0.002 mm to 0.006 mm ; 
the eutectic layer is, however, the most variable in thickness, 
varying from o to 0.022 mm. 

Fig. 13 shows the structure of the upper or corroded coating of 
1054. The eutectic layer has been corroded away, leaving only 
the alloy layer, which is generally, but not always, continuous. 

What has been said of the thickness of the constituent layers of 
the casting of 1054 allies generally to the other specimens 
examined, with the exception that the coating of 1054 seems to be 
much less uniform, both in thickness and structure than that of 
the others. Typical structures of the other samples are given in 
Figs. 7, 14, and 15. 

IV. ELECTROLYTIC POTENTIAL AND CORRODIBILITY OF THE CONSTIT- 
UENTS OF THE TIN COATING 

The properties of the alloys of copper and tin of high tin content 
have not been studied in great detail. Thurston, in a report 6 to 
the United States Board, account of which is also given in his 
book on the Materials of Engineering, 1890, gives results of physical 
tests of cast alloys throughout the whole range from o to 100 per 
cent copper. His results show that the alloys containing the con- 
stituent VIII are hard and very brittle. As soon as this constitu- 
ent disappears, the ductility increases. Apparently the con- 
stituent X.is hard, but not particularly brittle. The eutectic of 
tin and X. is, of course, soft and ductile. 

6 Executive Doc. 98, 45th Cong.; 1878-1881. 



Corrosion of Tinned Sheet Copper 1 1 

The constituents X and VIII are not readily attacked by dilute 
acids, even in the presence of mild oxidizing agents such as ferric 
chloride. Campbell 7 states that "when from i to 8 per cent of 
copper is present, casting produces a fine network of bright crystal- 
lites throughout the eutectic. On treatment with 10 per cent 
nitric acid and washing with dilute hydrochloric, the eutectic is 
dissolved, and a fine dark-brown powder is left behind, which 
seems to be composed of very small shapeless plates. * * * 
The various residues after treatment with dilute nitric acid be- 
come more and more coherent" (as the copper content increases). 
Heycock and Neville 8 isolated these constituents, VIII and X, 
by treatment of alloys containing them with concentrated hydro- 
chloric acid. 

Experiments were carried out to determine the comparative 
corrodibility of the various constituents of the tin coating. These 
were of two groups: Those in which attempt was made to deter- 
mine the electrolytic solution potential of the constituents and 
those in which actual, generally accelerated, corrosion tests were 
made. 

The difficulty of measuring directly the electrolytic emf's is at 
once apparent, since in the coating itself the constituents are so 
close together that in making a measurement only the resultant 
of the individual values is obtained, and this is practically equal to 
that of the most electropositive. Measurements 8 of the emf of 
the coating against that of the base copper gave practically the 
emf of pure tin against copper; that is, from + 260 to + 460 milli- 
volts, depending upon the electrolyte used. 

Samples were tested from which the tin or eutectic layer had 
been removed by boiling for a few minutes with concentrated 
hydrochloric acid. After such treatment the alloy layer, con- 
stituents VIII + X. remain. The emf against the base copper of 
the same sample was tested in each case. The results of typical 
measurements of this kind are given in the Table 2. Indication is 
here given that the alloy layer is electronegative to the copper. 
A positive value indicates that some tin has still remained in the 
coating. 

7 W. Campbell, The Microscopical Examination of the Alloys of Copper with Tin, Proc. Inst. Mech. 
Eng., 3-5, p. 1211; 1901. 

8 C. T. Heycock and E. H. Neville, The Constitution of the Copper- Tin Series of Alloys, Phil. Trans. 
Roy. Soc, 202, p. i; 1902. 

9 In all of such measurements the entire surface, except that to be tested, was protected from the elec- 
trolyte by a layer of paraffin. Further, each electrode was paraffined above and below the surface of the 
electrolyte, such that during the measurements every portion of the surface tested was completely 
immersed. 



12 



Technologic Papers of the Bureau of Standards 

TABLE 2 



The Electrolytic EMF Values of the "Alloy" Layers of the Tin Coating Against the 

Base Copper 



Time in 
minutes 


EMF in 

millivolts a 


Time in 
minutes 


EMF in 
millivolts a 


Time in 
minutes 


EMF in 
millivolts a 


1054 in 5 per cent H 2 SO< 


1054 against 4 A in tap 


1242 in dilute SnCla.HCl 


solution 


water 


solution 





-9.0 





>-80.0 





-55.0 


1 


-4.5 


36 


-57.0 


3 


-15.0 


5 


+4.5 


62 


-62.0 


8 


- 2.0 


8 


6-8.0 


103 


-70.0 


9 


d-12.0 


11 


c+4.0 






28 


± o 


1054 in tap water to 


1054 in 5 per cent HC1 




solution 


which has been 
added a few drops of 


1243 in dilute SnCl 2 .HCl 
solution 







1 

7 


-25.0 
-12.0 
+ 0.5 


SnCl2.HCl solution 







+50.0 






8 


d-22.0 





-35.0 


10 


+60.0 


10 


c+ 2.0 


7 
37 


-50.0 
—67.0 








1054 in 5 per cent HC1. 


51 


-52.0 




SnC)2 solution 


'1800 


« + 0.5 




6 


-60.0 




5 


-20.0 


1241 in dilute SnCl 2 .HCl 




18 


- 9.0 


solution 




20 
23 
65 
66 


d-50. 
c-22.0 
c+25.0 
d— 8.0 







15 


-15.0 
- 7.0 







° A plus sign indicates that the alloy layer was electro-positive to the copper in solution. 

6 Solution slightly stirred. 

c Solution quiet again. 

d Solution stirred. 

e A precipitate of basic tin chloride has been formed. 

In order to obtain a clearer indication of this, alloys were cast, 
using Banca tin and electrolytic copper, to have as nearly as 
possible the compositions of the constituents VIII and X, alloys 
1 1 24 (38 per cent Sn) and 1125 (59 per cent Sn), respectively. 
Since these alloys as cast do not consist wholly of these constitu- 
ents, they were homogenized by annealing for from 50 to 100 
hours just below 400 C. This produced two alloys, one of which 
consisted largely of VIII with traces of X and the eutectic; the 
other consisted largely of X with small grains of VIII and traces 
of eutectic. It may be mentioned that in such alloys there is 
considerable difficulty in getting rid by annealing of the eutectic, 
since it is absorbed in a peritectic reaction, which takes place very 
slowly. 



Corrosion of Tinned Sheet Copper 



TABLE 3 
EMF of Cast and Homogenized Copper-Tin Alloys to Copper 

[1125, 5g per cent; Sn, constituent X. 1124, 38 per cent; Sn, constituent VIII.] 



Time in 
minutes 



EMF in 
millivolts o 



1125 against annealed 
copper wire in dilute 
SnCh.HCl solution; 
surface completely 
paraffined 



b +.260 



1125 against annealed 
copper wire in 0.27 
N SnCh.N HC1 





1 

5 

31 



c-67 
-73 
-64 
-76 



1125 against electrolytic 
copper in 0.27 N SnCl 2 . 
N HC1; surface com- 
pletely paraffined 




2 

17 
34 



d - 9 

«-32 

-46 

-48 



1125 against electrolytic 
copper in 0.27 N SnCk. 
N HC1; surface com- 
pletely paraffined 




4 

25 
53 



/-17 
-30 
-35 
-42 



1124 against electrolytic 
copper in 0.27 N SnCk. 
N HC1; surface only 
ground and not paraf- 
fined 





5 

22 

24 
30 



-34 
-24 
-10 
-14 
-17 



Time in 
minutes 



EMF in 
millivolts a 



1125 against electrolytic 
copper in N/10 SnCl 2 . 
N HC1; surface 
completely paraffined 




37 



g -59 
-97 



1125 against electrolytic 
copper in N H2SO4; 
surface completely 
paraffined 




260 



h _67 
-40 



1124 against electrolytic 
copper in N/10 SnCl 2 . 
N/10 CUSO4. N HCi; 
surface completely 
paraffined 




3 

22 
27 



+9 
-2 
-5 
—5 



1125 against electrolytic 
copper in N/10 SnCl 2 . 
N/10 CuSQ 4 . N HCI 





9 

34 



i -17 
-14 
- 9 



1124 against electrolytic 
copper in N/1000 
SnCl 2 . N/1000 CUSO4. 
N/100 HCI; surface 
completely paraffined 




6 

19 



i± 

+17 
+ 12 



Time in 
minutes 



EMF in 
millivolts a. 



1125 against electrolytic 
copper in N H2SCV, 
surface completely 
paraffined 





6 

11 

49 



*' +26 
-13 
-23 
-12 



1124 against electrolytic 
copper in N H2SO4; 
surface completely 
paraffined 




5 

10 
12 



;+ 4 

- 6 

- 8 
fc-12 



1125 against 1241 copper 
base in tap water 




12 
24 



I +37 
+ 46 
+ 8 



1124 against electrolytic 
copper in N/1000 
SnCl 2 . N/100 HCI; 
surface completely 
paraffined 




480 



+ 



-45 



A plus sign indicates that the electrode first named 
is electropositive to the second one. 

*> Opening made to the eutectic. 

c Opening made by the VIII constituent. 

d Opening to X constituent. 

« Stirred solution. 

/ Several openings made, including grains of both 
VTII and X. 



g Opening to VHI only. 

^ Several openings made, including VIII and X. 

* Opening to VIII and X. 

;' Opening to VTn. 

k Stirred solution. 

I VIII+X. 



14 Technologic Papers of the Bureau of Standards 

The emf of these alloys was then measured against annealed 
copper wire or against electrolytic copper. A surface was pre- 
pared either by grinding alone, by grinding and polishing, or by 
grinding, polishing, and etching away the polished layer. In 
some cases the resultant emf of a portion of this surface was deter- 
mined; in others the whole surface was covered with paraffin, 
and then an opening was made with a sharp needle, exposing one 
or the other constituents only. This could be done under the 
microscope, as the grains of the constituent were relatively quite 
large. Typical results of such measurements are shown in Table 3. 
The measurements were made by potentiometer. 

The results indicate that the VIII and X constituents are both 
in general electronegative both to electrolytic copper and to 
remelted and worked copper, although when the concentration of 
Sn ions is very low they may give positive values. One set of 
tests made to determine the emf's of the individual constituents 
in the same alloy, 1 125, showed that the VIII was about 80 milli- 
volts and the X about 40 millivolts electronegative to copper. 
However, the variation in actual values obtained, not surprising 
in view of the difficult circumstances under which the tests were 
made, does not admit of any value being chosen for the emf of 
these constituents. Their values both lie close to that of copper, 
and in general below it by from 5 to 50 millivolts. 

This fact has remained hitherto unnoticed, although emf meas- 
urements of the copper-tin alloys have been made. 10 This is due 
to the fact that these measurements were carried out only on cast 
alloys, not subsequently homogenized by annealing. In such 
alloys containing between 36 and 100 atomic per cent tin, there 
remain always portions of the tin containing eutectic. The emf 
value obtained is therefore that of the tin; the electronegative 
values of the constituents VIII and X present are completely 
masked. 

The significance of the negative emf of these two constituents 
toward copper is realized when samples of tinned copper are 
exposed to corrosion. Two solutions and tap water were used 
in these tests, a solution of 

30 cc cone. HC1 
20 cc FeCl 3 .HCl solution, 
and a dilute solution of HN0 3 + HC1. Oblique surface sections 
were prepared of the samples 1054 and 1241, one in which the tin 

10 N. Puschin, Das Potential und die Chemische Konstitution der Metalllegierungen, Zeit. Anorg. Chem., 
66, p. 1, 1908; M. Herrschkowitsch, Bcitrag zur Kenntniss der Metalllegierungen, Zeit. Anorg. Chem., 
27, p. 123, 1898. 



Corrosion of Tinned Sheet Copper 1 5 

coating contains lead and one in which it did not, either as received 
or copper-plated. At first upon immersion in the FeCl 3 .HCl 
solution, and for about 15 minutes, the eutectic and tin layers were 
attacked and dissolved, bubbles of gas (hydrogen) forming on the 
exposed copper layers which remained perfectly bright and unat- 
tacked. As soon as the eutectic was dissolved the copper layers 
were quite suddenly attacked and darkened within three or four 
seconds, leaving the alloy layers bright and unetched. The 
appearance of specimen 1054 * s shown in Fig. 16. The islands of 
alloy containing black eutectic at the center are seen in a ground 
mass of heavily etched copper. The alloy layer will remain quite 
bright in such a solution for hours after both the tin and the copper 
have been heavily attacked. In HC1 + HN0 3 the alloy layer is 
attacked before the copper. 

A specimen of 1054 which had been plated was bent and polished 
through to the base copper and put in ordinary tap water for about 
24 hours. Both the eutectic layer and the copper were attacked, 
leaving the alloy layer bright as before. This is true also of other 
specimens treated for 200 or more hours. The appearance of the 
coating and adjacent copper of the 24-hour specimen is shown in 
the Fig. 17. The same was true of other samples, 1241 and 11 20, 
tested. 

V. DISCUSSION AND CONCLUSIONS 

It has been shown that the tin coating on copper consists of 
three well-defined layers — first and next to the copper a layer of 
the constituent VIII, probably the compound Cu 3 Sn, then of the 
constituent X, an alloy of approximately 60 per cent (by weight) 
of tin, and finally a layer of quite variable thickness, a eutectic of 
copper and tin (and probably also lead when this metal is used in 
the tinning mixture) , in which are found crystals of the constituent 
X. Etching experiments and measurements of electrolytic emf 
have indicated that these intermediate layers are electronegative 
to both the outer tin (eutectic) and the underlying copper itself 
(by from 5 to 50 miliovolts) , and less readily attacked by water 
and dilute acids (also alkalies) . This is true also of tin coatings 
containing lead, and holds not only for the corroded sheet exam- 
ined, 1054, but for all others examined, including several direct 
from the manufacturers. 

These results explain at once the local character and type of 
corrosion exhibited by the sample 1054, from the roof of the Library 
of Congress. The coating is very thin and- also quite variable in 



1 6 Technologic Papers of the Bureau of Standards 

thickness and structure. The surface scratches have exposed the 
copper at various points. This exposure of the copper along these 
scratches is aided by the fact that the adjacent alloy layer is 
extremely brittle and readily torn or crumbled out. As long as 
the tin eutectic layer was present it has owing to its greater corrod- 
ibility and electropotential been first attacked, thus protecting 
the copper. Finally, however, after several years this layer has 
been almost wholly removed, and at those points where the copper 
is exposed the attack has set in, the copper, forming with the 
adjacent electronegative alloy layer a galvanic couple, of which 
the copper is attacked and eaten away, forming the pits as 
described above. The same thing happens at the points where, 
as has been shown, there is a break in the continuity of the alloy 
layer. Here the eutectic layer is corroded off exposing the copper 
at once, the latter being corroded in similar manner as above 
described. 

Attention may be called here to the possible effect of the pres- 
ence of the small amounts of iron and zinc found in the coating. 
These metals are both electropositive to tin and do not dissolve 
appreciably in solid tin, must therefore be present in the 
coating as segregated particles. These must, in the case of the 
iron, at least, be very small since tests with a solution of dilute 
acid and K 3 FeC 6 N 6 , by which the presence of discrete particles of 
iron in a manganese bronze containing about i per cent of iron 
can be readily shown, fail to reveal them. 

It would thus appear that whenever the outer tin or eutectic 
layer of a tin coating on copper is removed the alloy layer remain- 
ing gives only a mechanical protection from corrosion — that is, it 
does not protect the underlying copper electrochemically, as does 
zinc, iron in galvanized products. Corrosion of the type 
described, therefore, should be possible in any tinned copper 
material. Yet instances are known of tinned copper roofs, which 
have stood up for 20 to 25 years under apparently more severe 
service conditions without showing sign of any such pitting as 
has been described. 

For the variation in resistance to corrosion of different samples 
of this material many factors might be responsible. First and 
foremost is the question of the mechanical abuses received, such 
as scratching and indenting. This has been shown to be the 
determining factor in the case described. A sample of the roof 
from the Statehouse in Texas showed absolutely no scratches; 
this roof has resisted corrosion for 20 or more years. 



Corrosion of Tinned Sheet Copper 1 7 

The other principal factor is undoubtedly that of the thickness 
and uniformity in structure of the coating. This varied quite 
noticeably in the various samples examined. The corroded sample, 
1054, showed perhaps the greatest degree of nonuniformity in this 
respect. 

A third factor which must not be lost sight of in this connection 
is that of the electrolytic solution potential of the base copper 
itself. Experiments have shown that this may vary for different 
samples within several millivolts, a range which is of the same 
order of magnitude as that of the difference in electromotive force 
between the copper and the tin-copper alloy. 

The author wishes to express his appreciation to the librarian 
of Congress, who brought the matter to the attention of the Direc- 
tor, Dr. Stratton, and to other officials of the Library of Congress 
for their cooperation in furnishing information and material, as 
well as to two manufacturers of this material, who have also 
furnished material of this type and information concerning it. 
Dr. Burgess, at whose direction the work was undertaken, has as 
usual been most ready with suggestion and helpful criticism, and 
to him and to Messrs. A. N. Finn and L. J. Gurevich, of this 
Bureau, who carried out the chemical analyses, the author's 
appreciation is expressed. 

Washington, July 12, 191 6. 



Photomicrographs 

[In the photomicrographs the bare copper area is marked (1), the electroplated copper (4)] 



Fig. 


Material 


Etching 


Magnifi- 
cation 


Description 


2 
4 

5 


1054 
1054 

1054 
1054 
1241 
1243 
1054 

1054 

1242 
1120 
1054 
1054 


NE^OH+E^Oa 

FeCl 3 .HCl+Cu-Am- 

OH-1. 
do 


lOp 
500 

100 
500 
100 
500 
500 

500 

100 

100 
100 
100 


Cross section of base copper. 
Transverse section of tin coating. 

Oblique surface section. 


6 


do 


Do. 


7 


do 


Do. 


8 


do 


Do. 


9-12 


do 


Transverse sections showing irregularity in thick- 


13 


do 


ness and structure of coating. 
Transverse section through coating remaining on 


14 


do 


corroded exposed side. 
Oblique surface section. 


15 


do 


Do. 


16 


FeCl 3 HCl 


Do. 


17 


Tap water 


Do. 









18 



e^ 



Bureau of Standards Technologic Paper No. 90 




Fig. i. — Appearance of corroded tinned sheet copper roof 



Bureau of Standards Technologic Paper No. 90 










Fig. 2. — Micro sir uciure of base copper 0/1054. X -Too 









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Fig. 4. — Material 1054. X 500 




Fig. 5. — Material 1054. X -Too 



Bureau of Standards Technologic Paper No. 90 




Fig. 6. — Material 1054. X S°° 




Fig. 7. — Material 1241. X 100 




Fig. 8. — Material J 2 43. X 500 



Bureau of Standards Technologic Paper No. 90 



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Fig . 9 . — Ma /erm J 1054 . X 500 




Fig. 10. — Material 1054. X 500 










Fig. 11.— Material 1054. X 500 



Bureau of Standards Technologic Paper No. 90 



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- ~i .p. V- 




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Fig. i2. — Material 1054. X 500 





Fig. 14. — Material 1242. X 100 



Bureau of Standards Technologic Paper No. 90 




Fig. 15. — Material 1120. X 100 




Fig. 16. — Material 1054. X 100 




Fig. 17. — Material 1054. X 100 



LIBRARY OF CONGRESS 



020 366 100 9 ♦ 



